The disclosure relates generally to wireless communications systems that support distributing communications services to remote units, and more particularly to reducing radio-frequency (RF) pass-band ripple in RF filters used for pass-band filtering of RF communications signals in wireless communication systems, including but not limited to distributed antenna system (DASs).
Wireless communication is rapidly growing, with ever-increasing demands for high-speed mobile data communication. As an example, local area wireless services (e.g., so-called “wireless fidelity” or “WiFi” systems) and wide area wireless services are being deployed in many different types of areas (e.g., coffee shops, airports, libraries, etc.). Distributed communications or antenna systems communicate with wireless devices called “clients,” “client devices,” or “wireless client devices,” which must reside within the wireless range or “cell coverage area” in order to communicate with an access point device. Distributed antenna systems are particularly useful to be deployed inside buildings or other indoor environments where client devices may not otherwise be able to effectively receive RF signals from a source, such as a base station for example. Example applications where DASs can be used to provide or enhance coverage for wireless services include public safety, cellular telephony, wireless local access networks (LANs), location tracking, and medical telemetry inside buildings and over campuses.
One approach to deploying a DAS involves the use of RF antenna coverage areas, also referred to as “antenna coverage areas.” Antenna coverage areas can be formed by remotely distributed antenna units, also referred to as remote units (RUs). The remote units each contain or are configured to couple to one or more antennas configured to support the desired frequency(ies) or polarization to provide the antenna coverage areas. Antenna coverage areas can have a radius in the range from a few meters up to twenty meters as an example. Combining a number of remote units creates an array of coverage areas. Because the antenna coverage areas each cover small areas, there typically may be only a few users (clients) per antenna coverage area. This arrangement generates a uniform high quality signal enabling high throughput supporting the required capacity for the wireless system users.
As an example, FIG. 1 illustrates distribution of communications services to coverage areas 10(1)-10(N) of a DAS 12, wherein ‘N’ is the number of coverage areas. These communications services can include cellular services, wireless services such as RFID tracking, Wireless Fidelity (WiFi), local area network (LAN), WLAN, and combinations thereof, as examples. The coverage areas 10(1)-10(N) may be remotely located. In this regard, the remote coverage areas 10(1)-10(N) are created by and centered on remote antenna units 14(1)-14(N) connected to a central unit 16 (e.g., a head-end controller or head-end unit). The central unit 16 may be communicatively coupled to a base station 18. In this regard, the central unit 16 receives downlink RF communications signals 20D from the base station 18 to be distributed to the remote antenna units 14(1)-14(N). The remote antenna units 14(1)-14(N) are configured to receive downlink communications signals 20D from the central unit 16 over a communications medium 22 to be distributed to the respective coverage areas 10(1)-10(N) of the remote antenna units 14(1)-14(N). Each remote antenna unit 14(1)-14(N) may include an RF transmitter/receiver (not shown) and a respective antenna 24(1)-24(N) operably connected to the RF transmitter/receiver to wirelessly distribute the communications services to client devices 26 within their respective coverage areas 10(1)-10(N). The remote antenna units 14(1)-14(N) are also configured to receive uplink RF communications signals 20U from the client devices 26 in their respective coverage areas 10(1)-10(N) to be distributed to the base station 18. The size of a given coverage area 10(1)-10(N) is determined by the amount of RF power transmitted by the respective remote antenna unit 14(1)-14(N), the receiver sensitivity, antenna gain and the RF environment, as well as by the RF transmitter/receiver sensitivity of the client device 26. Client devices 26 usually have a fixed maximum RF receiver sensitivity, so that the above-mentioned properties of the remote antenna units 14(1)-14(N) mainly determine the size of their respective remote coverage areas 10(1)-10(N).
With continuing reference to FIG. 1, the remote antenna units 14(1)-14(N) operate within a specific bandwidth in a specific RF spectrum or spectrums based on the supported communications services. This RF spectrum or spectrums is also known as pass-band frequency(ies). For instance, if a particular remote antenna unit 14 in the DAS 10 in FIG. 1 is configured to support Wi-Fi communications services, the remote antenna unit 14 may be configured to distribute downlink and uplink RF communications signals 20D, 20U in the pass-band between 2402 MHz and 2422 MHz in Industrial, Scientific, and Medical (ISM) band in the United States. Non-supported RF signals outside the pass-band may be suppressed to minimize interference to adjacent frequency bands. The client device 26 is configured to receive the downlink RF communications signals 20D in the pass-band frequency from the remote antenna units 14(1)-14(N) and suppress RF signals outside (e.g., above or below) the pass-band frequency so as to improve receiver sensitivity and performance. Also, the client device 26 is configured to transmit uplink RF communications signals 20U in a designated pass-band frequency to the remote antenna units 14(1)-14(N). Thus, RF transmitters and receivers in the remote antenna units 14(1)-14(N) can incorporate RF filters to pass desired downlink and uplink RF communications signals 20D, 20U within the pass-band frequency while attenuating unwanted RF communications signals outside (e.g., above or below) the pass-band frequency.
One type of RF filter that can be employed in the remote units 14(1)-14(N) to pass desired downlink and uplink RF communications signals 20D, 20U is a cavity RF filter. A cavity RF filter can provide high RF isolation to adjacent frequency bands of the pass-band and produce a relatively flat frequency magnitude response inside the pass-band. Another type of RF filter that can be employed in the remote antenna units 14(1)-14(N) to pass desired downlink and uplink RF communications signals 20D, 20U is a ceramic RF filter. A ceramic RF filter can also provide high RF isolation. Ceramic RF filters have cost and size advantages over cavity RF filters. However, a ceramic RF filter may suffer significant ripple in the pass-band frequency magnitude response compared to a cavity RF filter with the same bandwidth and out-of-band attenuation.
Ripple refers to fluctuations (measured in dB) in the pass-band of a RF filter's frequency magnitude response curve. In contrast to flat pass-band frequency magnitude response, ripple in a pass-band means that RF signals across the entire pass-band bandwidth will have different gains. For a downlink signal, some portions of the pass-band frequency signal will exhibit higher gain and therefore the downlink signal at these portions of the pass-band will be transmitted with higher power while other portions of the pass-band will exhibit lower gain and therefore the downlink signal at these portions of the pass-band will be transmitted with lower power. Having an equal gain across the entire pass-band bandwidth is important for getting the optimal performance. Because a RF transmitter's maximum transmit power is strictly limited by regulatory requirements, RF signals transmitted on frequencies with higher gains can maximize the output power without increasing the transmit power. RF coverage in the coverage areas 10(1)-10(N) in the DAS 12 in FIG. 1 will suffer as result of the uneven gains caused by ripple.
No admission is made that any reference cited herein constitutes prior art. Applicant expressly reserves the right to challenge the accuracy of any cited documents.